1. Field of the invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for controlling a transmit power in a wireless local area network.
2. Related Art
With recent development of information and communications technology, various wireless communications technology has been developed. Among them, a wireless local area network (WLAN) is technology that allows portable user equipment such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc. in a home, an enterprise or a certain service providing zone to have wireless access to high-speed Internet on the basis of radio frequency technology.
It is presupposed that communications in the WLAN based on institute of electrical and electronics engineers (IEEE) 802.11 standards are performed within a zone called a basic service set (BSS). The BSS zone has a somewhat indefinite boundary since it may vary depending on propagating characteristics of a wireless medium. Such a BSS is basically divided into two configurations of an independent BSS (IBSS) and an infrastructure BSS. The former indicates a BSS that forms a self-contained network and does not allows access to a distribution system (DS), and the latter indicates a BSS that includes one or more access points (AP), a distribution system, etc. and generally employs the AP in all communications including communication between stations.
The station (STA) having desire to access a wireless network may use two scanning methods for searching an accessible wireless network (BSS or IBSS), i.e., a candidate AP or the like.
One is passive scanning, which uses a beacon frame transmitted from the AP (or STA). That is, the STA having desire to access a wireless network periodically receives the beacon frames from the AP or the like managing a relevant BSS (or IBSS), thereby finding the accessible BSS or IBSS.
The other is active scanning. The STA having desire to access the wireless network first transmits a probe request frame. Then, the STA or AP that receives the probe request frame responds with a probe response frame.
TV Whitespace includes channels allocated to broadcast TV, which are permitted to be used by cognitive radio device. TV White Space may include UHF band and VHF band. The spectrum (hereinafter, can be called as ‘White Space’) not used by a licensed device can be used by an unlicensed device. The frequency band permitted to be used by unlicensed device can be differently defined for each country. Generally, this frequency band comprises 54-698 MHz (US, Korea), and some of this frequency band can't be used for the unlicensed device. Here, ‘licensed device’ means a device of the user permitted in this frequency band, and can be differently called as ‘primary user’, or ‘incumbent user’. The unlicensed device, which wishes to use the TV White Space (TVWS), shall acquire information for available channel list at its location.
An unlicensed device should provide a protection mechanism for the incumbent user. That is, the unlicensed device should stop using a specific channel, when an incumbent user, such as wireless microphone, is using that specific channel. For this purpose, spectrum sensing mechanism is required. Spectrum sensing mechanism comprises Energy Detection scheme, Feature Detection scheme, etc. By using this mechanism, unlicensed device determines that the channel is used by an incumbent user, when the strength of the primary signal is greater than a predetermined level, or when Digital Television (DTV) Preamble is detected. And, the unlicensed device (station or access point) shall lower its transmit power, when it is detected that the neighboring channel, next to the channel used by the unlicensed device, is used by the incumbent user.
On the other hand, in order to efficiently operate the unlicensed device on TVWS, more discussion is needed on an enabling mechanism of letting the unlicensed device to operate in TVWS, how efficiently the unlicensed device finds the network to be connected, how the information for the available channel in TVWS is efficiently acquired, efficient format of that information, and efficient signaling mechanism to exchange this information, etc.